Introduction to the resistivity surveying method. The resistivity of ...

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24 Another factor that the user can control is the size and distribution of the rectangular blocks used by the inversion model (Figure 15). By default, the program uses a heuristic algorithm partly based on the position of the data points to generate the size and position of the model blocks. The depth to the deepest layer in the model is set to be about the same as the largest depth of investigation of the datum points, and the number of model blocks does not exceed the number of datum points. In general, this produces a model where the thickness of the layers increase with depth, and with thicker blocks at the sides and in the deeper layers (Figure 16a). For most cases, this gives an acceptable compromise. The distribution of the datum points in the pseudosection is used as a rough guide in allocating the model blocks, but the model section does not rigidly follow the pseudosection. To produce a model with more uniform widths, the user can select a model where the number of model blocks can exceed the number of datum points (Figure 16b). Another possible configuration with model blocks of uniform thickness right up to the edges of the survey line is shown in Figure 16c. This is probably an extreme case. As the number of model blocks increase, the computer time needed to carry out the inversion also increases. This can be an important consideration for very large data sets with several hundred electrodes. Figure 16d shows the block distribution generated by a more quantitative approach based the sensitivity values of the model blocks. This technique takes into account the information contained in the data set concerning the resistivity of the subsurface for a homogeneous earth model. It tries to ensure that the data sensitivity of any block does not become too small (in which case the data set does not have much information about the resistivity of the block). The thickness of the layers can also be modified by the user. This can be used to extend the maximum depth of the model section beyond the depth of investigation of the data set. This is useful in cases where a significant structure lies just below the maximum depth of investigation of the data set. 2.7 Field examples Here we will look at a number of examples from various parts of the world to give you an idea of the range of practical survey problems in which the electrical imaging method has been successfully used. 2.7.1 Agricultural pollution - Aarhus, Denmark In many parts of Western Europe, which has large areas under cultivation, the contamination of water supplies due to fertilisers and pesticides used in agriculture has become a serious problem. A combined electromagnetic and resistivity survey was carried out the Department of Earth Sciences, University of Aarhus to map the lithology of the nearsurface unconsolidated sediments and aquifers in the Grundfor area at about 20 km northwest of Aarhus in Denmark (Christensen and Sorensen 1994). In this case, the concentration of the agricultural pollutants is very low and does not have a significant effect of the resistivity of the subsurface materials. The main purpose of the geophysical surveys was to map the lithology of the near-surface soil materials. Clayey soil is known to be relatively impermeable, but sandy soil which is relatively permeable can provide an infiltration path for the pollutants to enter the aquifers. Figure 16 shows the results from one of the resistivity surveys. The clayey soils, which have a relatively lower resistivity, can be easily distinguished from the sandy soils. The interpretation model from this survey was confirmed by a number of boreholes along the survey line. Copyright (1999-2001) M.H.Loke
25 2.7.2 Odarslov Dyke - Sweden A dolerite dyke surrounded by shales causes a prominent high resistivity zone (Dahlin 1996) near the middle of the pseudosection in the upper part of Figure 17. This is a particularly difficult data set to invert as the width of the high resistivity dyke is smaller than the depth to the lower section of the dyke. Thus the lower part of the dyke is less well resolved due to the reduction of the resolution of the resistivity method with depth. In the model section, the dyke shows up as a near vertical high resistivity body. This data set has 701 datum points and 181 electrodes. While the Wenner array is probably not the best array to map such a vertical structure, the dyke is still clearly shown in the model section. This survey was conducted in Sweden by Dr. Torleif Dahlin of the Department of Engineering Geology, Lund University. In the inversion of this data set, the robust inversion (Claerbout and Muir 1973) option in RES2DINV was used which sharpens the boundary between the dyke and the surrounding country rocks in the resulting inversion model. This choice is suitable for this data set since the dyke probably has a sharp boundary with the surrounding rocks. 2.7.3 Underground Cave - Texas, U.S.A This is an interesting example of a dipole-dipole survey to map caves within a limestone bedrock. This survey was carried out to map a previously known cave at the 4T Ranch north of Austin, Texas. This cave, which is filled with air, causes a prominent high resistivity anomaly near the centre of the pseudosection (Figure 18). The data was recorded with the Sting/Swift automatic multielectrode system manufactured by Advanced Geosciences, Inc. in Austin, Texas and the actual recording time was less than 40 minutes. In the course of this survey a new cave, subsequently named the Sting Cave, was discovered. This cave causes a high resistivity anomaly near the bottom left corner of the pseudosection. The inversion model gives the depth to the top of the Sting Cave at around 20 feet which agrees with the actual depth directly measured by an underground cave survey. This is a relatively small data set with 172 data points and 28 electrodes. A complete inversion took about 98 seconds (1.6 minutes) on a 90 Mhz Pentium computer, while on a 266 Mhz Pentium II computer it took only 23 seconds! Figure 16. (a) The apparent resistivity pseudosection for the Grundfor Line 2 survey with (b) the interpretation model section. Copyright (1999-2001) M.H.Loke
Page 1 and 2: Electrical imaging surveys for envi
Page 3 and 4: iii Table of Contents 1. Introducti
Page 5 and 6: v List of Figures Figure Page Numbe
Page 7 and 8: 1 1 Introduction to resistivity sur
Page 9 and 10: 3 carry out the more data intensive
Page 11 and 12: 5 2 2-D electrical imaging surveys
Page 13 and 14: 7 Note that as the electrode spacin
Page 15 and 16: 9 Figure 7. The apparent resistivit
Page 17 and 18: 11 function basically tells us the
Page 19 and 20: 13 Table 2. The median depth of inv
Page 21 and 22: 15 investigation) used in drawing t
Page 23 and 24: 17 electrode spacing along the surv
Page 25 and 26: 19 2.5.7 Summary If your survey is
Page 27 and 28: 21 significantly better results. Ho
Page 29: 23 Figure 15. Subdivision of the su
Page 33 and 34: 27 2.7.4 Landslide - Cangkat Jering
Page 35 and 36: 29 same depth with more data points
Page 37 and 38: 31 2.7.8 Marine bottom resistivity
Page 39 and 40: 33 collected at 5 hours after the p
Page 41 and 42: 35 2.7.11 Wenner Gamma array survey
Page 43 and 44: 37 sediment samples. The lower resi
Page 45 and 46: 39 Figure 29. The arrangement of th
Page 47 and 48: 41 columns of electrodes. 3.3 3-D r
Page 49 and 50: 43 The free 3-D resistivity forward
Page 51 and 52: 45 Figure 33. The models used in 3-
Page 53 and 54: 47 Figure 35. Horizontal and vertic
Page 55 and 56: 49 Figure 36. The model obtained fr
Page 57 and 58: 51 Figure 38. The 3-D model obtaine
Page 59 and 60: 53 Break 3 (No. 7), 16-20. Griffith
Page 61 and 62: 55 electrode is less than the x-loc
Page 63 and 64: 57 Figure 40. Inversion models for
Page 65 and 66: 59 of regions that are internally a
Page 67: 61 calculated apparent resistivity

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